Optical reception device

ABSTRACT

An optical reception device realizes stabilization of reception sensitivity inexpensively and highly precisely. The optical reception device includes: a Mach-Zehnder type 1-bit delay unit including a one-terminal input port and two-terminal output ports for decoding an optical difference phase shift keying (DPSK) signal and provided with one or more phase control functions to control the phase state of light; photoelectric conversion means for branching a portion of an optical output signal output from the one-side output port of the Mach-Zehnder type 1-bit delay unit, to convert the branched portion into an electric signal; and a phase control unit for controlling the phase state of the Mach-Zehnder type 1-bit delay device by using as an error signal the output signal of the photoelectric conversion means.

TECHNICAL FIELD

The present invention relates to an optical reception device, and moreparticularly, to an optical reception device using an optical decoder ofa Mach-Zehnder (MZ) type.

BACKGROUND ART

In a conventional optical communication system, for example, informationtransmission using a modulation system that utilizes light intensity,such as an OOK (On-Off Keying) system, is performed.

On the other hand, along with the recent realization of a high speedoptical communication system and reduction in its cost, a differentialphase shift keying (DPSK) system using optical phase information isattracting attention, because of being excellent in non-lineardurability and being expected as having an effect of improving thesensitivity to about twice (3 dB) as compared with the OOK system.

For decoding an optical signal transmitted in accordance with the DPSKsystem, an optical decoder using, for example, a 1-bit delay unit(hereinafter, referred to as “MZ-type 1-bit delay unit”) of theMach-Zehnder (MZ) type is used. The MZ-type 1-bit delay unit decodes anoptical signal transmitted using the electrooptical effect of adielectric waveguide formed of lithium niobate (LiNbO3) or the like.

In the MZ-type 1-bit delay unit, in a case where a phase differencebetween continuous bits is 0, an optical signal is output to one portcalled a constructive port, and in a case where the phase difference isπ, an optical signal is output to the other port called a destructiveport. Optical outputs from both the ports are received differentially atan optical receiver called a balanced receiver. Thus, in the opticalreception device having an MZ-type 1-bit delay unit, differentialreception using the balanced receiver is used, whereby the receptionsensitivity is enhanced.

However, in a case where the phase of the MZ-type 1-bit delay unit orthe wavelength of an input optical signal vary due to a change inenvironment such as temperature, the reception sensitivity is degraded.Thus, there is a conventional problem of stabilizing the receptionsensitivity even in the case where the environment such as temperaturevaries.

As a prior art for solving the above-mentioned problem, a process forstabilizing the phase of the MZ-type 1-bit delay unit is proposed, whichperforms control so that a bit rate frequency component of an opticalsignal received differentially at the balanced receiver becomes maximum(e.g., see Non-Patent Document 1).

The light reception device shown in Non-Patent Document 1 is composed ofa DPSK receiver including a 1-bit delay unit, a balanced receiver, andthe like, and a control system including an RF power branching unit, anRF power detector, a phase control circuit, and the like. A part of ahigh-speed electric signal immediately after the balanced receiver isbranched at the RF power branching unit so that the RF power thereof ismeasured by the RF power detector. As a result, in the optical receptiondevice, a phase control function of the Mach-Zehnder type 1-bit delayunit is controlled so that the measured RF power becomes maximum,whereby the reception sensitivity is stabilized.

Further, as one of the other processes, a process for stabilizing thephase of the MZ-type 1-bit delay unit that controls a phase based on aDC current flowing through the balanced receiver is also proposed (e.g.,see Non-Patent Document 2).

A reception device of an optical transmission system shown in Non-PatentDocument 2 is composed of a DPSK receiver including a 1-bit delay unit,a balanced receiver, and the like, and a control system including a DCcurrent detector, a phase control circuit, and the like. A DC currentcomponent of the balanced receiver is detected by a DC current detector,and a phase adjusting function provided in the Mach-Zehnder type 1-bitdelay unit is controlled based on the detected DC current value, wherebythe reception sensitivity is stabilized.

Non-Patent Document 1: Biljana Milivojevic et al, “Practical 40 Gbit/sCSRZ-DPSK transmission system with signed online chromatic dispersiondetection” ECOC2003, TU364

Non Patent Document 2: Eric A. Swanson et al, “High SensitivityOptically Preamplified Direct DitecOtion DPSK Receiver with ActiveDelay-Line Stabilization” Photonics technology letters, vol. 6, 1994

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

However, in the prior art shown in Non-Patent Document 1, there is aproblem in that expensive RF equipment such as an RF power branchingunit and an RF power detector are required.

On the other hand, in the prior art shown in Non-Patent Document 2,there is a problem in that, since control is performed based on the DCcurrent of the balanced receiver, the quality of an error signalrequired for the control is not sufficient.

Specifically, in a conventional optical reception device, it isdifficult to satisfy both the reduction in cost of the device and thecontrol precision for stabilizing an optical output.

The present invention has been made to solve the above-mentionedproblems, and an object of the present invention is to obtain an opticalreception device, which can be produced at low cost, and is capable ofstabilizing the fluctuation in reception sensitivity caused by thechange in phase of the MZ-type 1-bit delay unit and the change inwavelength of an input optical signal.

Means for Solving the Problems

An optical reception device according to the present invention includes:a Mach-Zehnder type 1-bit delay unit having one or a plurality of phasecontrol functions of controlling a phase state of light; photoelectricconversion means for branching a part of an optical output signal outputfrom a one-side output port of the Mach-Zehnder type 1-bit delay unitand converting it into an electric signal; and a phase control unit forcontrolling a phase state of the Mach-Zehnder type 1-bit delay unit byusing as an error signal an output signal of the photoelectricconversion means.

Effects of the Invention

The optical reception device according to the present invention controlsstabilization based on an output of optical/electric conversion means,so the optical reception device exhibits the effect of being able torealize the stabilization of reception sensitivity at low cost with highprecision.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a configuration of an opticalreception device according to Embodiment 1 of the present invention.

FIG. 2 is a view showing optical output characteristics with respect toa wavelength shift of an optical DPSK signal input to an input port ofan MZ-type 1-bit delay unit of the optical reception device according toEmbodiment 1 of the present invention and an optical signal output froman output port thereof.

FIG. 3 is an enlarged view of a spectrum in the vicinity of a centerwavelength (Δλ=0) in output characteristics of an output port 5 shown inFIG. 2.

FIG. 4 is an enlarged view of a spectrum in the vicinity of a centerwavelength (Δλ=0) in output characteristics of an output port 6 shown inFIG. 2.

FIG. 5 is a block diagram showing a configuration of an opticalreception device according to Embodiment 2 of the present invention.

FIG. 6 is a waveform diagram for explaining an operation at an optimumpoint in the optical reception device according to Embodiment 2 of thepresent invention.

FIG. 7 is a waveform diagram for explaining an operation when anoperation point shifts from an optimum value to a long wavelength sidein the optical reception device according to Embodiment 2 of the presentinvention.

FIG. 8 is a waveform diagram for explaining an operation when anoperation point shifts from an optimum value to a short wavelength sidein the optical reception device according to Embodiment 2 of the presentinvention.

FIG. 9 is a block diagram showing a configuration of an opticalreception device according to Embodiment 3 of the present invention.

FIG. 10 is a waveform diagram for explaining an operation at an optimumpoint in the optical reception device according to Embodiment 3 of thepresent invention.

FIG. 11 is a waveform diagram for explaining an operation when anoperation point shifts from an optimum value to a long wavelength sidein the optical reception device according to Embodiment 3 of the presentinvention.

FIG. 12 is a waveform diagram for explaining an operation when anoperation point shifts from an optimum value to a short wavelength sidein the optical reception device according to Embodiment 3 of the presentinvention.

FIG. 13 is a block diagram showing a configuration of an opticalreception device according to Embodiment 4 of the present invention.

FIG. 14 shows optical output characteristics of an output port 5 withrespect to a wavelength shift of an input optical signal under thecondition that an optical signal vs. noise ratio of an input opticalsignal is unsatisfactory in an MZ-type 1-bit delay unit of the opticalreception device according to Embodiment 4 of the present invention.

FIG. 15 is a diagram showing optical output characteristics of theoutput port 6 under the condition that an optical signal vs. noise ratioof an input optical signal in the MZ-type 1-bit delay unit of theoptical reception device according to Embodiment 4 of the presentinvention.

FIG. 16 is a block diagram showing a configuration of an opticalreception device according to Embodiment 5 of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, Embodiments 1 to 5 of an optical reception device of thepresent invention will be described in detail with reference to thedrawings. The present invention is not limited to Embodiments 1 to 5.

Embodiment 1

FIG. 1 is a block diagram showing a configuration of an opticalreception device according to Embodiment 1 of the present invention. Theoptical reception device shown in the figure includes a reception system(DPSK receiver) and a control system. The reception system includes anoptical input signal terminal 1, one-terminal input port 4, andtwo-terminal output ports 5 and 6, and is configured so as to include,for example, an MZ-type 1-bit delay unit 2 for decoding a DPSK signal orthe like, a balanced receiver 10 for differentially receiving an opticalsignal output from the two-terminal output ports 5 and 6, and anelectric output terminal 11. Further, the control system is configuredso as to include an optical coupler 7 for branching a part of an opticalsignal, a PD (Photo Diode) 8 as photoelectric conversion means, and aphase control unit 9 for outputting a phase control signal forcontrolling the phase of the MZ-type 1-bit delay unit 2.

The MZ-type 1-bit delay unit 2 has one or a plurality of phase controlfunctions 40 as an object to be controlled by the phase control unit 9.Regarding the phase control function 40, for example, electrodes (RFelectrode, DC electrode) provided in the MZ-type 1-bit delay unit 2 andan optical waveguide realize the functions thereof. For example, therefractive index of an optical waveguide can be changed based on avoltage applied to the electrodes, and the phase of an optical receptionsignal can be controlled based on the change in the refractive index.

Next, the operation of the optical reception device shown in FIG. 1 willbe described. In the figure, an optical signal output from the outputport 5 of the MZ-type 1-bit delay unit 2 is partially branched by theoptical coupler 7, and is converted into an electric signal by the PD 8.The electric signal converted by the PD 8 is input to the phase controlunit 9 for processing, and is applied to the phase control function 40of the MZ-type 1-bit delay unit 2 as a control signal. As describedlater, a case where the power of the electric signal becomes maximumcorresponds to an optimum operation point of the MZ-type 1-bit delayunit 2 with respect to the input optical signal, and by performingfeedback control so that the electric signal becomes a maximum value,the stabilization control of the reception sensitivity can be performed.

Note that in FIG. 1, a part of the optical signal of the output port 5is branched by the optical coupler 7. However, the optical output of theoutput port 6 of the MZ-type 1-bit delay unit 2 may be branched. Even inthe case of this configuration, the stabilization control of thereception sensitivity can be performed by the feedback control using anelectric signal subjected to photoelectric conversion.

FIG. 2 is a diagram showing optical output characteristics with respectto a wavelength shift of an optical DPSK signal input to the input port4 of the MZ-type 1-bit delay unit 2 and an optical signal output fromthe output ports 5 and 6. In the figure, a curve K1 indicated by thesolid line represents an optical spectrum of the DPSK signal input tothe MZ-type 1-bit delay unit 2, a curve K2 indicated by the wavy linerepresents a spectrum of an optical signal output from the output port 5(constructive port) of the MZ-type 1-bit delay unit 2, and a curve K3indicated by the alternate long and short dash line represents aspectrum of an optical signal output from the output port 6 (destructiveport) of the MZ-type 1-bit delay unit 2.

Further, FIGS. 3 and 4 are enlarged views of a spectrum in the vicinityof a center wavelength (Δλ=0) in each output characteristic of theoutput ports 5 and 6 shown in FIG. 2. For example, FIG. 3 shows aspectrum of the output port 5 (constructive port), and FIG. 4 shows aspectrum of the output portion 6 (destructive port). Both the graphs ofFIGS. 3 and 4 are normalized with each maximum value.

Next, an output stabilization control of the MZ-type 1-bit delay unit 2in the optical reception device shown in FIG. 1 will be described withreference to FIGS. 3 and 4. As shown in FIG. 3, the wavelength to beΔλ=0 provides a local maximum value, which becomes an optimum operationpoint of the phase control of the MZ-type 1-bit delay unit 2 withrespect to an input optical signal. Specifically, regarding the opticalsignal output from the output port 5, the maximum value of the opticaloutput characteristics with respect to a wavelength shift is controlled,whereby the reception sensitivity can be stabilized.

Similarly, as shown in FIG. 4, the wavelength to be Δλ=0 gives a localminimum value, which becomes an optimum operation point of phase controlof the MZ-type 1-bit delay unit 2 with respect to an input opticalsignal. Specifically, regarding the optical signal output from theoutput port 6, the minimum value control of the optical outputcharacteristics with respect to a wavelength shift is performed, wherebythe reception sensitivity can be stabilized.

Note that as a process for the maximum value control or the minimumvalue control of the optical output characteristics, a control algorithmsuch as a hill-climbing method can be used. Further, in a case ofcontrolling the stabilization of reception sensitivity at a high speed,for example, a control algorithm such as a gradient method or an optimumsolution search method based on a linear search method can be used.

As described above, according to the optical reception device accordingto Embodiment 1, the stabilization is controlled based on the outputfrom the photoelectric conversion means (e.g., PD), so the receptionsensitivity can be stabilized at low cost with high precision.

Embodiment 2

FIG. 5 is a block diagram showing a configuration of an opticalreception device according to Embodiment 2 of the present invention. Inthe optical reception device shown in the figure, the configuration of aphase control unit 9 is shown in detail. Specifically, the phase controlunit 9 shown in the figure is configured so as to include a preamplifier12 that receives an optical output of the PD 8, a low-pass filter (LPF)13 for blocking a high-frequency component of an output of thepreamplifier 12, a dither signal source 16 that generates a dithersignal, a mixer 14 serving as phase comparison means that receives thedither signal and the output signal of the LPF 13, and outputs as anerror signal a phase comparison signal obtained by comparing phasesbetween these output signals, a phase control circuit 15 that outputs aDC voltage based on the output signal (error signal) of the mixer 14,and an adder 17 that superimposes the dither signal on the DC voltage.The remaining configuration is the same as that of Embodiment 1, andthese components are denoted by the same reference numerals as thosethereof.

Next, the operation of the optical reception device shown in FIG. 5 willbe described. Herein, the processing different from that of Embodiment 1will be mainly described, and the description of the common processingwill be omitted.

In FIG. 5, the dither signal with a low frequency f [Hz] generated bythe dither signal source 16 is applied to the phase control function 40of the MZ-type 1-bit delay unit 2 via the adder 17. As a result, thedither signal is super imposed on optical signals output from the outputports 5 and 6. The low frequency indicates that a frequency is lowerthan that in an RF band, and the use of expensive RF equipment can beexcluded by using a signal with such a low frequency. Further, thedither signal is not required to be a sine wave with a single frequency,and may contain a frequency component with a low frequency f[Hz].

The dither signal superimposed on the optical signal is converted intoan electric signal by the PD 8 via the optical coupler 7 as lightintensity information. The dither signal converted into an electricsignal is amplified to a desired level by the preamplifier 12, passesthrough a low-pass filter 13 (hereinafter, abbreviated as “LPF”) havinga cut-off frequency of, for example, f[Hz], and is then output to oneinput terminal of the mixer 14. On the other hand, the other inputterminal of the mixer 14 receives a dither signal generated by thedither signal source 16. In the mixer 14 having received these signals,a phase comparison signal representing a phase comparison result betweenthe original dither signal and the dither signal after the photoelectricconversion is output as an error signal. In the phase control circuit15, a phase control signal based on the error signal is generated andoutput to the adder 17. The phase control signal is superimposed on thedither signal in the adder 17, and the phase of the MZ-type 1-bit delayunit 2 is controlled based on the phase control signal superimposed withthe dither signal.

Next, the operation of the optical reception device shown in FIG. 5 willbe described with reference to FIGS. 6 to 8. FIG. 6 is a waveformdiagram for explaining the operation at an optimum point in the opticalreception device shown in FIG. 5; FIG. 7 is a waveform diagram forexplaining an operation when an operation point shifts from an optimumvalue to a long wavelength side in the optical reception device shown inFIG. 5; and FIG. 8 is a waveform diagram for explaining an operationwhen an operation point shifts from an optimum value to a shortwavelength side in the optical reception device shown in FIG. 5.

FIG. 6 shows the operation of the MZ-type 1-bit delay unit 2 at anoptimum point. A phase control signal shown in (a) of the figurerepresents an output signal of the adder 17 which is applied to thephase control function 40 of the MZ-type 1-bit delay unit 2. A dithersignal with a low frequency f[Hz] generated by the dither signal source16 is superimposed on the phase control signal that is an output of thephase control circuit 15 via the adder 17.

The operation characteristic curve shown in (b) of the figure representsoptical output characteristics of the output port 5 (constructive port)of the MZ-type 1-bit delay unit 2. As represented by the operationcharacteristic curve, an optical output varies periodically with respectto a voltage applied to the phase control function 40. An operationpoint represented by a symbol “●” is an optimum operation point that cangive a maximum value of light intensity. Thus, in a case where a voltagecontaining a dither signal is applied, the output optical signal of theoutput port 5 (constructive port) of the MZ-type 1-bit delay unit 2contains a frequency component of 2 f[Hz] as shown in (c) of the figure,and the output signal from the preamplifier 12 has a waveform as shownin (d) of the figure.

The output signal of the preamplifier 12 and the dither signal shown in(e) of the figure output from the dither signal source 16 are subjectedto synchronous detection in the mixer 14, and the output signal of themixer 14 output at this time has a DC voltage at a zero level as shownin (f) of the figure.

FIG. 7 shows the operation of the MZ-type 1-bit delay unit 2 when avoltage applied to the phase control function 40 shifts from an optimumvalue to a long wavelength side. The phase control signal shown in (a)of the figure represents an output signal of the adder 17 applied to thephase control function 40 of the MZ-type 1-bit delay unit 2. The dithersignal with a low frequency f[Hz] generated by the dither signal source16 is superimposed on a control signal that is an output of the phasecontrol circuit 15 through the adder 17.

The operation characteristic curve shown in (b) of the figure representsoptical output characteristics of the output port 5 (constructive port)of the MZ-type 1-bit delay unit 2. As represented by the operationcharacteristic curve, the voltage applied to the phase control function40 shifts from the optimum value to the long wavelength side. As aresult, the output optical signal of the output port 5 (constructiveport) of the MZ-type 1-bit delay unit 2 contains a frequency componentof f[Hz] as shown in (c) of the figure, and the output signal of thepreamplifier 12 has a waveform as shown in (d) of the figure.

The output signal of the preamplifier 12 and the dither signal shown in(e) of the figure output from the dither signal source 16 are subjectedto synchronous detection in the mixer 14, and the output signal of themixer 14 output at this time becomes a negative DC voltage as shown in(f) of the figure.

FIG. 8 shows the operation of the MZ-type 1-bit delay unit 2 when thevoltage applied to the phase control function 40 shifts from an optimumvalue to a short wavelength side. (a) of the figure shows an outputsignal of the adder 17 applied to the phase control function 40 of theMZ-type 1-bit delay unit 2. The dither signal with a low frequency f[Hz]generated by the dither signal source 16 is superimposed on a controlsignal that is an output of the phase control circuit 15 through theadder 17.

The operation characteristic curve shown in (b) of the figure representsoptical output characteristics of the output port 5 (constructive port)of the MZ-type 1-bit delay unit 2. As represented by the operationcharacteristic curve, the voltage applied to the phase control function40 shifts from an optimum value to a short wavelength side. As a result,the output optical signal of the output port 5 (constructive port) ofthe MZ-type 1-bit delay unit 2 contains a frequency component of f[Hz]that is different in phase by a half circumference from the waveform of(c) of the figure, and the output electric signal of the preamplifier 12has a waveform as shown in (d) of the figure.

The output signal of the preamplifier 12 and the dither signal outputfrom the dither signal source 16 are subjected to synchronous detectionin the mixer 14, and the output signal of the mixer 14 output at thistime has a positive DC voltage as shown in (f) of the figure.

Thus, the mixer 14 outputs an error signal of the DC voltagecorresponding to the shift from the optimum value of the voltage appliedto the phase control function 40 of the MZ-type 1-bit delay unit 2.Accordingly, the phase control circuit 15 controls the phase controlfunction 40 of the MZ-type 1-bit delay unit 2 so that an error signaloutput to the phase control circuit 15 becomes zero, whereby thestabilization of the reception sensitivity can be realized.

Further, if a low-pass filter limiting the large range is provided inthe phase control circuit 15, an excess high-frequency componentcontained in the output of the mixer 14 can be suppressed.

Further, the optical coupler 7 shown in FIG. 5 may be placed in theoutput port 6 (destructive port) of the MZ-type 1-bit delay unit 2. Inthis case, when the voltage applied to the phase control function 40 ofthe MZ-type 1-bit delay unit 2 shifts from an optimum value to a longwavelength side, a positive error signal is obtained, and when thevoltage applied to the phase control function 40 of the MZ-type 1-bitdelay unit 2 shifts from an optimum value to a short wavelength side, anegative error signal is obtained, so the control opposite to thatdescribed above may be performed.

As described above, according to the optical reception device accordingto Embodiment 2, the stabilization control is performed using a dithersignal at low speed, in addition to the output of the photoelectricconversion means (e.g., PD). Therefore, the reception sensitivity can bestabilized at low cost with high precision.

Embodiment 3

FIG. 9 is a block diagram showing a configuration of an opticalreception device according to Embodiment 3 of the present invention. InEmbodiment 2, the dither signal to the preamplifier 12 is output fromany output port of the MZ-type 1-bit delay unit 2 via the opticalcoupler 7. However, Embodiment 3 is different from Embodiment 2 in thatthe dither signal is output from the balanced receiver 10. The remainingconfiguration is the same or equivalent to those of Embodiments 1 and 2,and the components thereof are denoted by the same reference numerals asthose thereof.

Next, the operation of the optical reception device shown in FIG. 9 willbe described. In the figure, the dither signal with a low frequencyf[Hz] generated by the dither signal source 16 is applied to the phasecontrol function 40 of the MZ-type 1-bit delay unit 2 via the adder 17.As a result, the dither signal is superimposed on optical signals outputfrom the output ports 5 and 6.

The dither signal superimposed on the optical signal is converted intoan electric signal by the balanced receiver 10 as light intensityinformation. The reception sensitivity of the dither signal convertedinto an electric signal is enhanced by 3 dB in a similar manner to thatof a DPSK signal by the differential reception of the balanced receiver10.

The dither signal converted into an electric signal is branched by theRF power branching unit 18, amplified to a desired level by thepreamplifier 12, passes through the LPF 13 having a cut-off frequency off[Hz], for example, and is then output to one input terminal of themixer 14. On the other hand, the dither signal generated by the dithersignal source 16 is input to the other input terminal of the mixer 14.The mixer 14 that receives those signals outputs a phase comparisonsignal as an error signal, which represents a phase comparison resultbetween the original dither signal and the optically/electricallyconverted dither signal. In the phase control circuit 15, the phasecontrol signal based on the error signal is generated and output to theadder 17. The dither signal is superimposed on the phase control signalin the adder 17, and the phase control of the MZ-type 1-bit delay unit 2is executed based on the phase control signal superimposed with thedither signal.

Next, the operation of the optical reception device shown in FIG. 9 willbe described with reference to FIGS. 10 to 12. FIG. 10 is a waveformdiagram for explaining the operation at an optimum point in the opticalreception device shown in FIG. 9; FIG. 11 is a waveform diagram forexplaining the operation when the operation point shifts from an optimumvalue to a long wavelength side in the optical reception device shown inFIG. 9; and FIG. 12 is a waveform diagram for explaining the operationwhen the operation point shifts from an optimum value to a shortwavelength side in the optical reception device shown in FIG. 9.

FIG. 10 shows the operation of the MZ-type 1-bit delay unit 2 at anoptimum point. The phase control signal shown in (a) of the figurerepresents an output signal of the adder 17 which is added to the phasecontrol function 40 of the MZ-type 1-bit delay unit 2. The dither signalwith a low frequency f[Hz] generated by the dither signal source 16 issuperimposed on the phase control signal that is an output of the phasecontrol circuit 15 via the adder 17.

The operation characteristics curve shown in (b) of the figurerepresents optical output characteristics of the output port 5(constructive port) of the MZ-type 1-bit delay unit 2 and the outputport 6 (destructive port) thereof. According to these operationcharacteristics, an optical output changes reciprocally and periodicallywith respect to a voltage applied to the phase control function 40, andeach optical output becomes a maximum output (output port 5) and aminimum output (output port 6) at each optimum point represented by asymbol “●” of (b) of the figure. Thus, in the case where a voltagecontaining a dither signal is applied, the respective output opticalsignals of the output ports 5 and 6 of the MZ-type 1-bit delay unit 2contain a frequency component of 2 f[Hz] with phases inverted from eachother as shown in (c) of the figure, and the output signal of thepreamplifier 12 obtained after the differential reception forms awaveform as shown in (d) of the figure.

The output signal of the preamplifier 12 and the dither signal shown in(e) of the figure output from the dither signal source 16 are subjectedto synchronous detection in the mixer 14, and the output signal of themixer 14 output at this time becomes a DC voltage at a zero level asshown in (f) of the figure.

FIGS. 11 and 12 each show waveforms representing the respectiveoperations in the case where the voltage applied to the phase controlfunction 40 shifts from an optimum value to a long wavelength side, orin the case where the voltage applied to the phase control function 40shifts from an optimum value to a short wavelength side. The basicoperations are the same as those of FIGS. 7 and 8, except that thedifferential output signals obtained in FIGS. 11( d) and 12(d) haveamplitudes twice those of FIGS. 7( d) and 8(d).

Thus, the mixer 14 outputs an error signal of a DC voltage correspondingto a shift from an optimum value of the voltage applied to the phasecontrol function 40 of the MZ-type 1-bit delay unit 2. Thus, the phasecontrol circuit 15 controls the phase control function 40 of the MZ-type1-bit delay unit 2 so that the error signal output to the phase controlcircuit 15 becomes zero, whereby the reception sensitivity can bestabilized. Further, since the differential output signal is an electricsignal having a two-fold amplitude, the effect of expecting theenhancement of the ability of detecting an error of 3 dB as comparedwith Embodiment 2, can be obtained.

As described above, according to the optical reception device accordingto Embodiment 3, in the same manner as in Embodiment 2, thestabilization is controlled using a dither signal with a low frequency,so the stabilization of reception sensitivity can be realized at lowcost, and a dither signal is detected after the differential reception.Thus, the stabilization of reception sensitivity can be realized withhigher precision.

Embodiment 4

FIG. 13 is a block diagram showing a configuration of an opticalreception device according to Embodiment 4 of the present invention. Theoptical reception device shown in the figure is configured so as toinclude an optical filter 19 which is inserted between the optical inputsignal terminal 1 and the MZ-type 1-bit delay unit 2, in addition to theconfiguration of Embodiment 1. The remaining configuration is the sameor equivalent to that of Embodiment 1, and the components thereof aredenoted by the same reference numerals as those thereof.

The optical filter 19 has a band of about 0.8 to 2 times that of a freespectrum range of the MZ-type 1-bit delay unit 2. Thus, in the opticalreception device of Embodiment 4, an unwanted noise component is reducedby the optical filter 19, and the band width corresponding to the freespectrum range of the MZ-type 1-bit delay unit 2 is selected, wherebythe ability of detecting an error after optical/electrical signalconversion can be enhanced, and the precision of the stabilization ofreception sensitivity can be increased.

Further, FIG. 14 is a diagram showing optical output characteristics ofthe output port 5 with respect to a wavelength shift of an input opticalsignal under the condition that an optical signal vs. noise ratio of theinput optical signal is unsatisfactory in the MZ-type 1-bit delay unit 2shown in FIG. 13. FIG. 15 is a diagram showing optical outputcharacteristics of the output port 6 under the condition.

Next, the enhancement of the ability of detecting an error signal byusing the optical filter 19 in the optical reception device shown inFIG. 13 will be described with reference to FIGS. 14 and 15.

In FIG. 14, a curve represented by a symbol “◯” of the figure shows anoptical output of the output port 5 of the MZ-type 1-bit delay unit 2when the optical filter is not used, and a symbol “●” of the figureshows an optical output when the optical filter 19 having a band twicethat of a free spectrum range of the MZ-type 1-bit delay unit 2 is used.

Further, Δλ=0 of FIG. 14 corresponds to an optimum operation point ofthe phase of the MZ-type 1-bit delay unit 2 with respect to the inputoptical signal. As represented by the waveform of the figure, it can beunderstood that the change ratio of an optical output with respect to ashift from an optimum operation point where Δλ=0 is enhanced. For thisreason, the ability of detecting an error after optical/electricalsignal conversion can be enhanced and the precision of the stabilizationof reception sensitivity can be increased by using the optical filter19.

Further, even in the optical output characteristics of the output port 6shown in FIG. 15, the point where Δλ=0 becomes an optimum operationpoint that gives a local minimum value, and the change in an opticaloutput can be enhanced with respect to the shift from the optimumoperation point where Δλ=0 in the same way as in the output port 5.

As the optical filter 19, one having a band that is 0.8 times that ofthe free spectrum range of the MZ-type 1-bit delay unit 2 may beselected. In the case of using the optical filter 19 with suchcharacteristics, the change ratio of an optical output can be furtherincreased with respect to the shift from the optimum operation pointwhere Δλ=0, with hardly influencing the optical output characteristicsin the vicinity of Δλ=0.

As described above, according to the optical reception device ofEmbodiment 4, the optical filter 19 having a band of about 0.8 to 2times that of the free spectrum range of the 1-bit delay unit is used.Therefore, the reception sensitivity can be stabilized at low cost withhigh precision.

Note that Embodiment 4 is achieved by adding the optical filter 19,which is inserted between the optical input signal terminal 1 and theinput port 4 of the MZ-type 1-bit delay unit 2, to the configuration ofEmbodiment 1. However, this configuration can also be added to theconfigurations of Embodiment 2 and 3, and Embodiment 5 described later.Even in these configurations, the same effects as those of Embodiment 4can be obtained.

Embodiment 5

FIG. 16 is a block diagram showing a configuration of an opticalreception device according to Embodiment 5 of the present invention. Inthe optical reception device shown in the figure, in the configurationof Embodiment 2 shown in FIG. 5, a clock data recovery circuit(hereinafter, abbreviated as “CDR circuit”) 20 that functions as errordetection means and an FEC decoder 21 are added to a reception system,and a second phase control system including a phase control circuit 22that generates and outputs a control signal for controlling the MZ-type1-bit delay unit 2 based on the error detection results of the FECdecoder 21 is added to a control system, in addition to the first phasecontrol system including the optical coupler 7, the PD 8, and the phasecontrol unit 9 shown in FIG. 5. The remaining configuration is the sameor equivalent to that of Embodiment 2, and the components thereof aredenoted by the same reference numerals as those thereof.

Next, the operation of the optical reception device shown in FIG. 16will be described. Herein, the processing different from that ofEmbodiment 2 will be mainly described, and the description of the commonprocessing will be omitted.

In FIG. 16, the DPSK signal is converted into an electric signal at thebalanced receiver 10. The converted electric signal is subjected toidentification reproduction processing in the CDR circuit 20, andthereafter, subjected to error correction processing in the FEC decoder21. At this time, an error signal based on the error correctioninformation in the FEC decoder 21 is generated. The phase controlcircuit 22 controls the phase control function 41 of the MZ-type 1-bitdelay unit 2 by using the error signal based on the error correctioninformation.

Thus, the second phase control system according to Embodiment 5 useserror correction information of a main signal as an error signal, so thestabilization of the reception sensitivity can be realized with highprecision.

Further, if the phase control of the MZ-type 1-bit delay unit 2 isperformed using the first phase control system and the second phasecontrol system as control loops of different time constants, both thephase control systems perform operations complementary to each other, sothe stability of the control can be enhanced.

Further, by using the control in the first phase control system forcoarse adjustment, and by using the control in the second phase controlsystem for fine adjustment, both the stabilization of the control andthe increase in speed of the control can be satisfied.

As described above, according to the optical reception device accordingto Embodiment 5, the stabilization control using a dither signal at alow speed in addition to the output of the photoelectric conversionmeans (e.g., PD) is performed, and the control using the errorcorrection information of the reception system (DPSK receiver) is alsoperformed. Therefore, the reception sensitivity can be stabilized at lowcost with high precision, and the stabilization control of receptionsensitivity can be performed at a high speed.

As described above, the present invention is useful as a receptiondevice applicable to a high-speed optical transmission system, and inparticular, preferable in a case where it is desired to realize thestabilization of reception sensitivity easily or the like.

1. An optical reception device, comprising: a Mach-Zehnder type 1-bitdelay unit having one or a plurality of phase control functions ofcontrolling a phase state of light, the Mach-Zehnder type 1-bit delayunit having first and second optical output ports; photoelectricconversion means for branching a part of an optical output signal outputfrom the first optical output port of the Mach-Zehnder type 1-bit delayunit and converting it into an electric signal; a phase control unitthat produces a phase control signal on the basis of the electric signaloutput from the photoelectric conversion means, the phase control unitcomprising: phase comparison means for comparing phases of a componentof the electric signal output from the photoelectric conversion meansand a dither signal, and generating an error signal representing theresult of the comparison, a phase control circuit that converts theerror signal from the phase comparison means into a DC voltage, and anadder that produces the phase control signal by superimposing the dithersignal onto the DC voltage, the phase control signal being sent to theMach-Zehnder type 1-bit delay unit to control a phase state of theMach-Zehnder type 1-bit delay unit; and a balanced receiver thatdifferentially receives the optical output signals from the first andsecond output ports, and produces therefrom an electric output signal.2. An optical reception device according to claim 1, wherein the phasecontrol unit further comprises: a dither signal source from which thedither signal is sent to the phase comparison means and the adder; and alow pass filter for passing a low frequency component of the electricsignal output from the photoelectric conversion means before theelectrical signal is sent to the phase comparison means; wherein thephase state of the Mach-Zehnder type 1 -bit delay unit is controlledbased on an output signal from the adder.
 3. An optical reception deviceaccording to claim 2, wherein a DC voltage is output as the error signaloutput from the phase comparison means.
 4. An optical reception deviceaccording to claim 2, wherein the dither signal contains a low-frequencysignal component that has a low frequency as compared with a frequencyin an RF band.
 5. An optical reception device according to claim 2,wherein a cut-off frequency of the low-pass filter is set to be afrequency of a low-frequency signal component contained in the dithersignal.
 6. An optical reception device, comprising: a Mach-Zehnder type1-bit delay unit having one or a plurality of phase control functions ofcontrolling a phase state of light; photoelectric conversion means forbranching a part of an optical output signal output from the firstoptical output port of the Mach-Zehnder type 1-bit delay unit andconverting it into an electric signal; a phase control unit thatproduces a first phase control signal, the first phase control signalbeing sent to the Mach-Zehnder type 1-bit delay unit to control a phasestate of the Mach-Zehnder type 1-bit delay unit, the first phase controlsignal being produced on the basis of the electric signal output fromthe photoelectric conversion means, the phase control unit comprising: adither signal source that generates a dither signal, a low-pass filterfor passing a low-frequency component of the electric signal output fromthe photoelectric conversion means, phase comparison means for comparingphases of the low-frequency component of the electric signal passed fromthe low-pass filter and the dither signal generated by the dither signalsource, and outputting as an error signal a phase comparison signal thatrepresents the result of the phase comparison, a first phase controlcircuit that converts the error signal output from the phase comparisonmeans into a DC voltage, and an adder that produces the first phasecontrol signal by superimposing the dither signal generated by thedither signal source onto the DC voltage; a balanced receiver fordifferentially receiving an optical output signals output from the firstand second output ports of the Mach-Zehnder type 1-bit delay unit, andproduces therefrom an electric output signal; error detection means forgenerating an error signal based on error correction informationobtained when identification reproduction processing and errorcorrection processing are performed with respect to a differentialoutput of the balanced receiver; and a second phase control circuit forgenerating a second phase control signal for controlling theMach-Zehnder type 1-bit delay unit based on an output of the errordetection means, wherein the phase control unit controls a phase stateof the Mach-Zehnder type 1-bit delay unit based on the first phasecontrol signal from the adder, and wherein the second phase controlcircuit controls a phase state of the Mach-Zehnder type 1-bit delay unitbased on the second phase control signal.
 7. An optical reception deviceaccording to claim 1, further comprising an optical filter connected toa front stage of an input port of the Mach-Zehnder type 1-bit delayunit, and having a band of 0.8 to 2 times that of a free spectrum rangeof the Mach-Zehnder type 1-bit delay unit.